Edges become sharp after flip normal
Your mesh has 211 double vertices that mess up the shading.

Select all vertices (Ctrl+A), disable Autosmooth and go:


Mesh > Clean up > Merge by distance (this will merge 211 double vertices)
Mesh > Normals > Recalculate outside
RMB > Shade Smooth


This will ensure you have a clean mesh, normals pointing out and proper shading.